Development of a hard x-ray focal plane compton polarimeter: a compact polarimetric configuration with scintillators and Si photomultipliers

Abstract

X-ray polarization measurement of cosmic sources provides two unique parameters namely degree and angle of polarization which can probe the emission mechanism and geometry at close vicinity of the compact objects. Specifically, the hard X-ray polarimetry is more rewarding because the sources are expected to be intrinsically highly polarized at higher energies. With the successful implementation of Hard X-ray optics in NuSTAR, it is now feasible to conceive Compton polarimeters as focal plane detectors. Such a configuration is likely to provide sensitive polarization measurements in hard X-rays with a broad energy band. We are developing a focal plane hard Xray Compton polarimeter consisting of a plastic scintillator as active scatterer surrounded by a cylindrical array of CsI(Tl) scintillators. The scatterer is 5 mm diameter and 100 mm long plastic scintillator (BC404) viewed by normal PMT. The photons scattered by the plastic scatterer are collected by a cylindrical array of 16 CsI(Tl) scintillators (5 mm×5 mm×150 mm) which are read by Si Photomultiplier (SiPM). Use of the new generation SiPMs ensures the compactness of the instrument which is essential for the design of focal plane detectors. The expected sensitivity of such polarimetric configuration and complete characterization of the plastic scatterer, specially at lower energies have been discussed in [11,13]. In this paper, we characterize the CsI(Tl) absorbers coupled to SiPM. We also present the experimental results from the fully assembled configuration of the Compton polarimeter.

Hard X-ray Polarimetry -- An overview of the method, science drivers and recent findings

“…Consistent with this discussion, we have seen a lot of efforts in the last few years to develop Rayleigh polarimeters in 10 − 30 keV range (Paul et al, 2016) and Compton polarimeters at higher energies both as focal plane detector (Chattopadhyay et al, 2013(Chattopadhyay et al, , 2014aChattopadhyay et al, 2015;Krawczynski et al, 2011) and large area of scintillators or CZTs or Germanium detectors (McConnell et al, 2009;Chattopadhyay et al, 2014b;Vadawale et al, 2015;Orsi & Polar Collaboration, 2011;Caroli et al, 2012Caroli et al, , 2018Yang et al, 2018). In the next section, we will review instrumentation of different scattering polarimetry configurations suitable in the three energy bands: 10 − 25 keV, 25 − 80 keV and beyond 80 keV.…”

“…In recent years, multiple groups across the globe have started investigating possible implementation of Compton scattering based X-ray polarimeter coupled with NuSTAR type of hard X-ray optics, e.g. X-Calibur (Guo et al, 2013;Beilicke et al, 2014), CX-POL (Chattopadhyay et al, 2013(Chattopadhyay et al, , 2014bChattopadhyay et al, 2015), PolariS (Hayashida et al, 2014) scattering efficiency and low probability of multiple interactions in the scattering volume. This type of instruments can achieve ∼1 % MDP level in 1 Ms exposure for 100 mCrab sources assuming NuSTAR type of focusing area.…”

“…X-Calibur uses an array of 2×2×0.2 cm 3 CZT detectors (64 pixels) on four sides of the plastic scatterer. CXPOL (Chattopadhyay et al, 2015) and PolariS (Hayashida et al, 2014), on the other hand, use long CsI(Tl) and GSO scintillator bars respectively to detect the scattered X-rays. Use of CZT detector array provides 2D positional information of the scattered photons which can be used to extract the polar scattering angle (if plastic is vertically position sensitive) and therefore can be used to further filter out the chance background events.…”

“…Though SiPMs have been mostly used at higher energies (Bloser et al, 2013;Sanaei et al, 2015), lower energy (20 − 80 keV) application of SiPMs was demonstrated only recently during CXPOL experiment (Chattopadhyay et al, 2015) to readout CsI(Tl) scintillators with SiPM PM3350 from KETEK 7 . One major issue of SiPMs is the high background level (∼500 kHz/mm 2 ), therefore use of SiPMs at lower energies requires very good light collection at the SiPM which will depend on the scintillator light output and decay time constant.…”

Gamma-Ray Polarimetry

“…The first flown focal-plane scattering polarimeter was the balloon-borne mission X-Calibur (Beilicke et al, 2014;Guo et al, 2013;Kislat et al, 2018), which will be succeeded by an improved version called XL-Calibur to be launched for the first time in 2022 (Abarr et al, 2021). Other implementations of the principle include the Japanese small satellite PolariS (Polarimetry Satellite, Hayashida et al, 2014) and the Compton X-ray Polarimeter (CXPOL, Chattopadhyay et al, 2013Chattopadhyay et al, , 2014aChattopadhyay et al, 2015) currently under consideration for the next Indian astronomy satellite. All of these polarimeters consist of a compact low-Z scattering element surrounded by high-Z photon detectors, but details of the implementation vary.…”